1. Field of the Invention
The invention relates to displacement sensors and, particularly, to a micro displacement sensor based on photonic crystals.
2. Description of Related Art
A micro displacement sensor is important for a micro-electron-mechanical system (MEMS). The micro displacement sensors are configured to precisely measure a relative displacement between elements in MEMS. Additionally, the micro displacement sensors are widely used in varied devices, such as biosensor or atomic force microscopy (AFM).
Because photonic crystals have photonic band gaps (PBGs), micro displacement sensors based on the photonic crystals are extensively being developed. Micro displacement sensor based on photonic crystals can have a high sensitivity ranging from about 1 a to about 1.6 a (“a” being a lattice constant), even though a measuring range for such a sensor is within the range of less than 1.5 a. In addition, a micro displacement sensor based on a photon tunneling effect and a Fano interference effect can obtain a 20 dB transmission contrast when a relative displacement changes by about 1% of operating wavelength. Moreover, a micro displacement sensor based on a defect resonant cavity in a photonic crystal can provide a sensitivity ranging from about 1 a to about 1.15 a, even though a measuring range for such a sensor is within the range of from −0.55 a to 0.60 a.
However, the measuring range for displacement provided by the micro displacement sensors mentioned above is limited. That is, it is difficult to obtain a micro displacement sensor with a wide dynamic range for displacement measurement. Particularly, it is difficult to measure a relative displacement ranging over twofold order of the lattice constant by such micro displacement sensors.
What is needed, therefore, is a micro displacement sensor having a large dynamic range of displacement measurement.